The present invention relates to a protective device for a load connected to a power supply line of an electric power system. More particularly, the present invention relates to a protective device including an LC network having low-pass characteristics and connected into the power supply line before a load.
Protective devices of this type, which can also be called line filters, are used for keeping disturbances present on the power supply line away from the load, for example a sensitive electronic device. A large number of various designs of such protective devices are known. However, a problem common to all these embodiments is that only high-frequency disturbances in the radio-frequency range are sufficiently well suppressed. Thus, the lower cut-off frequency of the known line filters is rarely much below 50 khz. Known protective devices do not adequately suppress disturbances with lower frequencies or ramp-shaped rises such as are caused, for example, by post-arcing currents of lightning, switching surges or by slow electro-magnetic pulses (this particularly refers to MHD EMP's, the magneto-hydrodynamic electromagnetic pulses occurring as a delayed consequence of the ignition of nuclear explosive charges above the layer of atmosphere of the earth), H. W. Zaininger, Electromagnetic Pulse (EMP) Interaction with Electric Power Systems; Report ORNL/Sub.-82-47905/1, August 1984, Zainiger Engineering Company, 3408 Vance Court, San Jose, Calif. 95132. Although passive LC networks having arbitrarily low cut-off frequencies can be theoretically implemented by using sufficiently large capacitances and inductances, the achievable lower cut-off frequency, in practice, has a value of approximately the aforementioned value of 50 kHz. The reason for this is, on the one hand, that the magnitude of the capacitances is limited for safety reasons, particularly for reasons of contact protection. On the other hand, the magnitude of the inductances is also limited from the point of view of the practicality of their physical size.
The present invention achieves the object of specifying a protective device of the type initially mentioned which can also be used for keeping disturbances with frequencies below the cut-off frequency of the LC network safely away from the load.
Essentially, the advantages of the invention can be seen in the fact that the object aimed at by the invention is achieved without using capacitances which are doubtful from the point of view of safety and unmanageably large and extensive inductances.